1. Field of the Invention
The present invention relates to a method of driving a mask stage and a method of mask alignment. More particularly, the present invention relates to a method of driving a stage, which is suitably applied to a case wherein a reticle-side stage is driven in a scan direction in a slit-scan exposure type projection exposure apparatus, and a method of mask alignment in the projection exposure apparatus.
2. Related Background Art
When a semiconductor element, a liquid crystal display element, a thin film magnetic head, or the like is manufactured in a photolithography process, a projection exposure apparatus for transferring a pattern on a photomask or a reticle (to be generally referred to as a "reticle" hereinafter) onto a substrate (a wafer, glass plate, or the like) coated with a photosensitive material is used.
As a conventional projection exposure apparatus, a step-and-repeat type reduction projection exposure apparatus (stepper) for sequentially exposing a pattern image on a reticle onto each of shot areas by sequentially moving the shot areas of a wafer into an exposure field of a projection optical system is popularly used.
In recent years, since patterns on semiconductor devices or the like tend to be miniaturized, it is required to increase the resolution of a projection optical system. For this reason, in order to increase the resolution, a technique for decreasing the wavelength of exposure light, a technique for increasing the numerical aperture of the projection optical system, and the like have been examined. However, with either technique, it becomes difficult to maintain high accuracy of imaging performance (a distortion, curvature of field, and the like) on the entire exposure field when an exposure field as large as that in the prior art is to be assured. For this reason, an apparatus which is currently reconsidered its use is a so-called slit-scan exposure type projection exposure apparatus.
In the slit-scan exposure type projection exposure apparatus, a pattern on a reticle is exposed onto a wafer, wherein the reticle and wafer are being synchronously scanned relative to a rectangular or arcuated illumination area (to be referred to as a "slit-like illumination area" hereinafter).
Therefore, when a pattern with the same area as that in the stepper system is to be exposed onto a wafer, the exposure field of the projection optical system in the slit-scan exposure system can be set to be smaller than that in the stepper system. As a result, accuracy of imaging performance in the exposure field may be improved.
The mainstream of the conventional reticle size is 6", and the mainstream of the projection magnification of the projection optical system is .times.1/5. However, as the area of the circuit pattern of, e.g., a semiconductor element increases, the 6" reticle cannot serve its purpose at the .times.1/5 magnification. For this reason, a projection exposure apparatus in which the projection magnification of the projection optical system is changed to, e.g., .times.1/4 must be designed. In order to cope with such an increase in area of a pattern to be transferred, the slit-scan exposure system is advantageous.
In a projection exposure apparatus of this type (stepper), a reticle must be aligned in advance on a reticle stage. For this purpose, a reticle alignment device is arranged on a reticle mark on the reticle. Such a reticle alignment device is disclosed in U.S. Pat. No. 4,710,029. In an alignment system disclosed in U.S. Pat No. 4,710,029, light reflected by an alignment mark on a reticle is incident on a sensor via a vibration mirror and a slit. When the output from the sensor is synchronously detected by a driving signal of the vibration mirror, the position of the alignment mark relative to a slit is detected. The position of the alignment mark is detected based on a signal from the sensor in the alignment system, and the reticle is moved by a servo system, so that the alignment mark accurately coincides with the slit. As a result, alignment of the reticle with respect to the apparatus main body is executed.